WHAT IS a CV Axle?

CV (constant-velocity) axles, (also known as half-shafts), are used in front-wheel drive vehicles to transfer the engine’s power from the transaxle to the two drive wheels.  Some late model vehicles with independent rear suspension and some four-wheel and all-wheel drive vehicles also use CV axles.







A CV axle has two CV joints (an inner joint and an outer joint).  These joints allow the axle to transfer the engine's power to the drive wheels at a constant speed while accommodating for the different travel conditions, including the up and down motion of the suspension and cornering.  Without constant-velocity joints, the axles would bind or break whenever the vehicle hit a bump in the road or turned a corner.

CV joints are packed with grease for lubrication.  To keep the grease in the joint and moisture, dirt and roadway grime out, the joint is covered with a rubber boot called a CV boot.  CV boots are made of a durable rubber that can withstand extreme weather and travel conditions.  Good quality CV boots can easily last 100,000 miles or more.  But, because they are constantly exposed to the elements, a CV joint boot can be damaged at any time.  For this reason, it is a good idea to routinely inspect your CV boots for damage.  If your vehicle has more than 125,000 miles - it is a good idea to check the CV joints more often.

How CV Axles Fail

Actually, CV axles rarely fail - it's the CV joint(s) that fail and they typically only fail because of a damaged or torn CV boot.  When a CV boot fails due to age and wear, or is damaged by roadway debris, the grease that keeps the joint lubricated quickly leaks out of the joint and dirt, roadway grime and moisture enters the joint. When this occurs, the CV joint will soon fail.










Of the two joints, the outer CV joint almost always fails first because it does more work than the inner joint so it wears out sooner.  The outer joint is also much more likely to be damaged from roadway debris because of its location.  For these reason, if you suspect CV-joint failure, odds are it is the outer joint that has failed.  (See above image)

NOTE: While a damaged or torn CV boot is almost always to blame for a failed CV joint, like any part, CV joints can wear out over time.  As the joint ages and wears, tight tolerances give way to added space opening up between the moving parts of the joint.  As space between the parts increases the parts begin to bang against each other and after awhile they break.


Symptoms of a Bad or Failing CV Joint

A bad or failing CV joint will oftentimes make a clicking, popping or grinding noise, especially when turning.  At low speed, a bad CV joint may make a steady knocking sound every couple of seconds or so.  A bad CV joint can also cause a vibration that is felt in the steering wheel and floorboard of the vehicle - this may happen only at certain speeds.

INSPECTING a CV Axle and Checking/Testing a CV Axle

A routine inspection of the CV axles and CV boots and checking/testing a CV axle you suspect of being bad are two different processes.   Below is an explanation.








Routine CV Axle Inspection

Inspecting a CV axle involves a visual check of the CV boots.  In most instances, all you will need is a flashlight.

Follow these Steps...

1 Park the vehicle on level ground and set the emergency brake.  If necessary, lift the front of the vehicle and secure with jack stands.  
1 With your flashlight in hand, slide underneath the vehicle. If you've raised the front of the vehicle, you can slide underneath at the front center, which will enable you to see and inspect both CV axles.  Or, when there is less room underneath the vehicle to maneuver, slide underneath one side of the vehicle immediately in front of or behind the front wheel to inspect one axle at a time.  
1 Inspect the CV Joint Boots.  When you are in a position to see one or both CV axles, look closely at the CV boots located at each end of the axle. (Each axle has two CV boots).  Visually inspect each rubber boot for cracks and tears.  Also check for a damaged or missing boot clamp.  For any areas of the boot you cannot see, use you finger to feel for the presence of grease.  A damaged boot will expel grease onto the outside of the boot and surrounding area.

The axle passes the inspection if no visual crack or tear is seen or felt in either boot and there is no presence of grease on the outside of either boot.

Checking/Testing a CV Axle You Suspect of Being Bad

Checking and testing a CV axle you suspect of being bad or beginning to fail involves a visual inspection, a movement test and a drive test.

Follow these Steps...

1 Follow the steps above to inspect the CV axle and CV boot for damage.
If one of your CV axles is showing symptoms of being bad or going bad, you will probably see a damaged or torn CV boot when you do the above inspection.  However, regardless of what you find during the inspection of the CV boots, if you still suspect you have a bad CV axle/joint, you will need to do a feel and drive test to confirm.
1 Allow engine to cool.  From underneath the front of the vehicle, grab the CV axle with a firm grip and attempt to move sideways and then in and out.  The CV axle should have very minimal movement in any direction.  If you are able to move the axle more than 1/8 inch in any direction, it is probably time to replace the axle.  
1 Take the vehicle for a drive.  To get an accurate reading of the CV axle condition, find a smooth flat surface (such as an empty parking lot) and turn the steering wheel all the way in one direction and drive in a circle.  Then turn the steering wheel in the opposite direction and drive in a circle.  If one of the CV joints is bad, you will hear a popping or banging noise as you travel in a circle.

WARNING: If you hear a CV axle popping or clicking when traveling straight, the CV joint is near the end of its service life and will completely fail soon.  At this stage the CV axle must be replaced soon.


What is a CV-Axle/Joint?
SYMPTOMS of a Bad CV Joint